1. Field of the Invention
This invention relates to ink jet printing devices, and more particularly to larger thermal ink jet printheads which are fabricated by an anisotropic etching technique that utilizes predetermined selective mask undercutting to provide printheads that are robust without sacrificing resolution.
2. Description of the Prior Art
Thermal ink jet printing is a type or drop-on-demand ink jet systems, wherein an ink jet printhead expels ink droplets on demand by the selective application of electrical pulses to thermal energy generators, usually resistors, located one each in capillary-filled, parallel ink channels a predetermined distance upstream from the channel nozzles or orifices. The channel end opposite the nozzles are in communication with a small ink reservoir to which a larger external ink supply is connected.
U.S. Re. 32,572 to Hawkins et al. discloses a thermal ink jet printhead and several fabricating processes therefor. Each printhead is composed of two parts aligned and bonded together. One part is a substantially flat substrate which contains on the surface thereof a linear array of heating elements and addressing electrodes, and the second part is a silicon substrate having at least one recess anisotropically etched therein to serve as an ink supply manifold when the two parts are bonded together. A linear array of parallel grooves are also formed in the second part, so that one end of the grooves communicate with the manifold recess and the other ends are open for use as ink droplet expelling nozzles. Many printheads can be made simultaneously by producing a plurality of sets of heating element arrays with their addressing electrodes on a silicon wafer and by placing alignment marks thereon at predetermined locations. A corresponding plurality of sets of channel grooves and associated manifolds are produced in a second silicon wafer. In one embodiment, alignment openings are etched in the second silicon wafer at predetermined locations. The two wafers are aligned via the alignment openings and alignment marks, then bonded together and diced into many separate printheads.
U.S. Pat. No. 4,638,337 to Torpey et al. discloses an improved thermal ink jet printhead similar to that of Hawkins et al., but has each of its heating elements located in a recess. The recess walls containing the heating elements prevent the lateral movement of the bubbles through the nozzle and therefore the sudden release of vaporized ink to the atmosphere, known as blow-out, which causes ingestion of air and interrupts the printhead operation whenever this event occurs. In this patent a thick film organic structure such as Riston.RTM. or Vacrel.RTM. is interposed between the heater plate and the channel plate. The purpose of this layer is to have recesses formed therein directly above the heating elements to contain the bubble which is formed over the heating elements, thus enabling an increase in the droplet velocity without the occurrence of vapor blow-out and concomitant air ingestion.
U.S. Pat. No. 4,774,530 to Hawkins discloses the use of patterned thick film insulative layer to provide the flow path between the ink channels and the manifold, thereby eliminating the fabrication steps required to open the channel groove closed ends to the manifold recess, so that the printhead fabrication process is simplified.
U.S. Pat. No. 4,786,357 to Campanelli et al., discloses the use of a patterned thick film insulative layer between mated and bonded substrates. One substrate has a plurality of heating element arrays and addressing electrodes formed on the surface thereof and the other being a silicon wafer having a plurality of etched manifolds, with each manifold having a set of ink channels. The patterned thick film layer provides a clearance space above each set of contact pads of the addressing electrodes to enable the removal of the unwanted silicon material of the wafer by dicing without the need for etched recesses therein. The individual printheads are produced subsequently by dicing the substrate having the heating element arrays.
As disclosed in the above-discussed patents, thermal ink jet printheads are fabricated from two substrates. One substrate contains the heating elements and the other contains ink recesses. When these two substrates are aligned and bonded together, the recesses serve as ink passageways. A plurality of each substrate is formed on separate wafers, so that the wafers may be aligned, mated, and diced into many individual printheads. The wafer for the plurality of sets of recesses is silicon and the recesses are formed by an anisotropic etching process. The anisotropic or orientation dependent etching has been shown to be a high yielding fabrication process for precise, miniature printheads. They are low cost, high resolution, electronically addressable printers with high reliability. Such printheads are usually about a quarter of inch wide and print small swaths of information being translated across a stationary recording medium such as paper. The paper is then stepped the distance of one swath and the printing process continued until the entire page of paper is printed. This is a low speed process.
In efforts to increase the printing speed, larger arrays of nozzles are required. Each ink droplet emitting nozzle requires an ink channel which is in communication with an ink reservoir or manifold. In order to complete the etching from only one side of the wafer, the reservoir is etched through the wafer so that the open bottom may serve as an ink inlet. As the array size increases, so also does the reservoir and thus the ink inlet. As the area of the through etch for the reservoirs increase, the wafer strength diminishes and yield drops because many of the fragile wafers are damaged during subsequent assembly operations.
While the anisotropic etching process has many attributes, one of its drawbacks is that a very restricted set of geometries are available because the {111} etch termination planes form a pyramid with the {100} plane as a base. Therefore, only squares and rectangular shapes can be produced in the {100} surface plane, and perpendicular to the {100} plane, pyramidal pits are formed. The square etch pits can be pointed, or rectangular pits can come to an edge if the etch process is allowed to continue until full {111} plane termination occurs, or the bottom of the pit can remain a {100} plane parallel to the surface, if etching is not complete. Of course, if the square or rectangular vias in the etch resistant mask is large enough relative to its thickness, the square or rectangular etched recess will etch through and be open at the bottom with the recess walls being {111} planes.
For silicon printheads, anisotropic or orientation dependent etching of silicon wafers makes use of the preferred etching of the {100} planes to {111} planes. This etch rate ratio can be greater than 100:1. As discussed above, a silicon wafer is coated with a material that is inert to the anisotropic etch bath, such as, for example, a silicon nitride masking layer in an etch bath of potassium hydroxide (KOH). This coating of etch resistant material, usually silicon nitride, is resist coated, photo-patterned, and plasma etched to define a pattern of vias in the silicon nitride. The wafer is then placed in an etchant, resulting in the recesses which have {111} crystal plane walls. Depending upon the size of the vias and the time in the etchant, V-grooves and through holes are formed. Critical to successful orientation dependent etching is alignment of the via patterns to the {111} plane, since any rotation from it results in enlarged etched recesses, as disclosed in more detail in copending application D/88240 by Hawkins et al., entitled "Large Monolithic Thermal In Jet Printheads" , and assigned to the same assignee as this invention. Closely adjacent vias can, therefore, cause the etched recesses to merge destroying the intended design. This invention deals with these orientation dependent etching problems while enabling larger printheads to be fabricated without increase in the fragility of the etched wafers.